Nematodes taking big bite out of cotton profits

• When it comes to nematodes, growers need to forget about getting rid of them and focus their attention on managing the tiny subterranean pests that can devastate a cotton crop before it gets started.
• Damage from nematodes in cotton often goes unnoticed or is attributed to a host of other diseases, nutritional or insect-related damage.

There is little doubt there will be plenty of cotton in the Southeast in 2012 and a good bet that prices will remain around the dollar a pound mark.

Taking care of the cotton crop in most soils in the Upper Southeast starts with taking care of an easy-to-miss problem — nematodes.

When it comes to nematodes, growers need to forget about getting rid of them and focus their attention on managing the tiny subterranean pests that can devastate a cotton crop before it gets started.

Nematodes are microscopic worm-like animals that feed primarily on the roots of cotton plants. Plant-parasitic nematodes puncture root cells with a sharp stylet and withdraw nutrition from the host plant.

The Society of Nematologists contends nematodes do about $3 billion in crop damage on an annual basis worldwide. They represent four out of every five animals on earth. They’ve caused as much as $81 million worth of damage annually to crops in a single state in the U.S.

Damage from nematodes in cotton often goes unnoticed or is attributed to a host of other diseases, nutritional or insect-related damage.

“The first three things growers need to do in managing nematodes are sample, sample, sample,” says long-time Clemson University Plant Pathologist John Mueller.

“If you don’t know what nematodes you have and how many you have, none of the tools we can recommend will mean much,” Mueller says.

“Saying you have a nematode problem in a cotton field is much like saying you have an insect problem — without knowing which one, it’s hard to manage it,” he adds.

Soil volume is critical when evaluating the results from a nematode sample. Some states express their results per pint of soil. Other states express their results “per 100 milliliters of soil”. Your local Extension agent or crop consultant can be sure the thresholds you use to interpret your results are for the proper soil volume.

Mueller, who doubles as Director of the Edisto Agricultural Research and Extension Center, says South Carolina and surrounding states have root-knot and reniform nematodes that are common in many Southeast states, but they also have Columbia lance nematodes, which aren’t nearly so common.

Completely different beast

Columbia lance nematodes are much different than the two more commonly occurring pests. It is a migratory endoparasite, and it feeds in an entirely different manner than root-knot and reniform nematodes, Mueller says.

“We’re not likely to find resistance to Columbia lance nematodes. It’s going to be tough to rotate around and generally is a very difficult pest for farmers who find it in their soils,” the South Carolina researcher adds.

In the Upper Southeast in 2012, there is likely to be a significant increase in peanut production and a less dramatic increase in cotton. That could be really good news for growers battling nematode problems.

None of the three most common nematodes found in cotton (root-knot, reniform and Columbia lance) feed on peanut root systems. Sting nematodes, a fourth species that occurs in cotton in the Southeast may or may not feed on peanuts, depending on where the population occurs.

The flip side of rotation is soybeans. Every nematode that goes to cotton also feeds extensively on soybeans. “If you rotate cotton and soybeans, all you are doing is creating more problems,” Mueller says.

Corn is a mixed bag with nematodes. Growers need to know precisely which nematodes are causing the problem, if they are rotating cotton and corn. If Southern root-knot is the primary nematode, corn in the rotation won’t do any good.

If reniform nematodes are the big problem, corn is a good solution and may provide two years of control for one year of corn in the rotation, Mueller says.

How much reduction in nematodes can be achieved by crop rotation is subject to many, many crop and soil conditions and often comes down to a field by field analysis by the grower or his or her crop consultant.

In a seven-year series of tests in the Southeast, USDA researchers found an increase of 26 percent in yield for cotton and 10 percent greater in peanuts, versus growing either crop in a continuous rotation.

Using other tools, like resistant varieties in combination with nematicides would clearly increase yields even more in some situations.

In the USDA tests, which were conducted primarily in Georgia, using chemical treatments had little impact on cotton yields, but increased peanut yields by 22 percent above the yield gain achieved by rotating the two crops.

Determining how many nematodes it takes to cause yield damage to crops is a bit like shooting a BB gun at a hummingbird — it’s an elusive target to say the least.

More damage on sandy soils

In general the sandier a soil is the more damage nematodes will cause. This is due to the moisture stress already present in the soil. Nematodes prune the root system making it difficult for the plant to take up the moisture which is already in short supply.

Results from a test by Scott Monfort, when he worked in Arkansas, give a good example of this principle.

In his test a cotton crop grown in a soil with 40 percent sand could sustain a two bale cotton crop even when 2,000 Southern root-knot nematodes per 100 ml. of soil were present. In the area of the field with 60 percent sand just 200 nematodes per 100 ml. of soil would significantly reduce yield.

Monfort is currently Extension peanut specialist at Clemson University and works with Mueller at the Edisto Agricultural Research and Extension Center in Blackville, S.C.

Irrigation is another factor that affects nematode damage thresholds. The combination of scenarios is almost endless when you add irrigation into the mix, Mueller says.

Variables aside, the general rule of thumb is to classify nematode populations in a field as either above or below the damage threshold levels.

“If they are above the damage threshold we would estimate at least a 10 percent yield loss,” Mueller says

“There is a second threshold above which we would expect a yield loss of 20 percent or more. In fields which exceed the low threshold we have recommended low rates of Temik 15G in the past.

“With a shortage of Temik 15G we now recommend the seed treatments. For populations above the high threshold we have recommended combinations of seed treatments plus Temik 15G or using Telone II.

“In some cases where nematode populations are too high to control or yield potentials for a field are low we strongly recommend to growers they rotate to a non-host crop to get the nematode situation under control, Mueller says.

For Southern root-knot nematodes it usually takes at least 100 nematodes per 100 milliliters of soil to cause a yield loss.

For Columbia lance nematodes, the low threshold is 80 and the high threshold is 175 per 100 milliliters of soil.

For reniform nematodes, the low threshold is 250 and the high 625 or more.

For sting nematodes 1 per sample is the standard threshold.

“A deep, dark secret of nematology is that these numbers aren’t plus or minus five percent — maybe 20 percent or more depending upon when the sample was taken and what levels of stress are present in a field,” Mueller says.

“To be more accurate, individual growers should use their knowledge of individual fields to adjust thresholds,” he adds.

“When I see a high threshold, I think the grower is looking at 25 percent or more yield loss. In these fields we can justify the $40-50 per acre for Telone II that is needed to make a profitable crop,” he says.

Nematodes are an easy-to-miss killer of crop yields and the damage they cause is even easier to dismiss and underestimate.

The key to manage these tiny yield killers is knowledge: Know what specie is in your field and sample to determine how many, then make an informed decision as to how to most profitably manage the problem.